The present disclosure relates to chemical-mechanical planarizing machines and methods to maintain processing pads and other planarizing media.
Mechanical and chemical-mechanical planarizing processes (collectively xe2x80x9cCMPxe2x80x9d) remove material from the surface of semiconductor wafers, field emission displays or other microelectronic workpieces in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a carrier assembly 30, and a planarizing pad 40. The CMP machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the planarizing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow F), or it reciprocates the platen 20 back and forth (indicated by arrow G). Since the planarizing pad 40 is attached to the under-pad 25, the planarizing pad 40 moves with the platen 20 during planarization.
The carrier assembly 30 controls and protects the workpiece 12 during planarization. The carrier assembly 30 generally has a workpiece holder 32 to pick up, hold and release the workpiece 12 at appropriate stages of the planarizing process, or the workpiece 12 may be attached to a resilient pad 34 in the holder 32. The holder 32 may be a free-floating wafer carrier, or an actuator assembly 36 may be coupled to the holder 32 to impart axial and/or rotational motion to the workpiece 12 (indicated by arrows H and I, respectively).
The planarizing pad 40 and a planarizing solution 44 on the pad 40 collectively define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the workpiece 12. The planarizing pad 40 can be a soft pad or a hard pad. The planarizing pad 40 can also be a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution 44 is typically a non-abrasive xe2x80x9cclean solutionxe2x80x9d without abrasive particles.
To planarize the workpiece 12 with the CMP machine 10, the carrier assembly 30 presses the workpiece 12 face-downward against the polishing medium. More specifically, the carrier assembly 30 generally presses the workpiece 12 against the planarizing solution 44 on a planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier assembly 30 moves to rub the workpiece 12 against the planarizing surface 42. As the workpiece 12 rubs against the planarizing surface 42, material is removed from the face of the workpiece 12.
In the highly competitive semiconductor industry, it is desirable to maximize the throughput of CMP processing by producing a planar surface on a workpiece as quickly as possible. The throughput of CMP processing is a function, at least in part, of the polishing rate of the workpiece assembly and the ability to accurately stop CMP processing at a desired endpoint. The polishing rate is a function of several factors, many of which may change during planarization. For example, the condition of the planarizing surface on the planarizing medium can affect the polishing rate. Typically, the polishing rate for a fixed-abrasive pad decreases after planarizing 3 to 10 workpieces. Changes in the polishing rate can also occur at other, unexpected times during planarization thereby reducing the accuracy of stopping a planarizing cycle at a desired endpoint and reducing the consistency of planarity of the workpieces. Therefore, it is generally desirable for CMP processes to provide (a) a uniform polishing rate across the face of a workpiece to enhance the planarity of the finished workpiece surface, and (b) a reasonably consistent polishing rate during a planarizing cycle to enhance the accuracy of determining the endpoint of a planarizing cycle.
CMP processes should consistently and accurately produce a uniformly planar surface on the workpiece to enable precise fabrication of circuits and photo-patterns. During the construction of transistors, contacts, interconnects and other features, many workpieces develop large xe2x80x9cstep heightsxe2x80x9d that create highly topographic surfaces. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing microelectronic devices on a workpiece.
One factor affecting the uniformity of the workpiece surface is the condition of the planarizing pad. The planarizing surface of the pad can deteriorate after polishing a number of workpieces because waste matter from the workpieces, planarizing solution and/or the pad accumulates on the planarizing surface. The planarizing surface can also deteriorate because rubbing the workpiece against the pad alters the planarizing surface of the pad in a manner that may produce inconsistent results in uniformity. The wear characteristics on the pad, for example, depend upon the density pattern of the workpiece because different types of workpieces produce different wear characteristics on the planarizing surface of the pad.
The effects of workpiece wear on fixed-abrasive pads are particularly problematic. A high density workpiece typically has more topographical variations on the active side of the workpiece than a low density workpiece; therefore, a high density workpiece more aggressively wears the pad than a low density workpiece. As such, the polishing rate for a run of high density workpieces may not drop significantly after planarizing several workpieces. On the other hand, low density workpieces do not aggressively wear the pad surface, and thus they often xe2x80x9cpassivatexe2x80x9d the planarizing surface of the pad. This can quickly reduce the polishing rate of low density workpieces. Therefore, different planarizing pads are generally used to planarize different types of workpieces and/or products in fixed-abrasive CMP. Changing the pad for each type of workpiece, however, is time-consuming and reduces the throughput of using fixed-abrasive pads.
One conventional technique to decrease the variability of CMP processing is xe2x80x9cconditioningxe2x80x9d the pad to restore the surface of the pad to a consistent state. Non-abrasive planarizing pads are conventionally conditioned with devices that rub an abrasive element on the planarizing surface. For example, one method for conditioning non-abrasive pads is to abrade the planarizing surface with a diamond end-effector. Another method to condition fixed-abrasive or non-abrasive pads involves agitating the pad-slurry-wafer interface using ultrasound to prevent the accumulation of particulate matter on the pad.
U.S. Pat. No. 6,083,085 issued to Lankford discloses a conditioning device for conditioning planarizing media. The conditioning device has a support assembly with a support member and a conditioning head attached to the support member. The support member may be a pivoting arm that carries the conditioning head over the planarizing medium. The conditioning head may have a non-contact conditioning element that transmits a form of non-contact energy to waste matter on the planarizing medium. For example, the non-contact conditioning element can be a mechanical-wave transmitter that transmits mechanical waves that act against waste matter on the planarizing pad to break the bonds between the planarizing medium and the waste matter. U.S. Pat. No. 5,895,550 issued to Andreas discloses a method and apparatus for chemical mechanical polishing that includes an acoustic energy source positioned to transmit acoustic energy into a polishing slurry to break up agglomerated particles in the slurry before the polishing slurry contacts the wafer surface. U.S. Pat. No. 5,245,790 issued to Jerbic discloses a chemical-mechanical polishing apparatus that includes an ultrasonic transducer mounted to the underside of a platen that introduces mechanical vibratory energy against the pad or into the slurry during polishing. Jerbic, more specifically, discloses that the frequency of the transducer is selected to be approximately two or more orders of magnitude higher than the rotational frequency of the platen.
Although the devices and methods disclosed in the above-referenced patents are useful for overcoming certain problems regarding the variability of the planarizing pads, these patents do not address other problems associated with planarizing different types of workpieces. For example, these patents do not address the problems associated with changing the pads for planarizing different types of workpieces on a single CMP machine. These patents also do not address the problems associated with fluctuations in the polishing rate during a planarizing cycle of a workpiece. Thus, it would be desirable to develop a method and apparatus for (a) processing different types of workpieces on the same pad, and (b) preventing fluctuations in the polishing rate during a planarizing cycle.
The present invention is directed toward chemical-mechanical planarizing machines and methods to maintain processing pads and other planarizing media used in planarizing microelectronic workpieces. In one embodiment of the invention, a method for planarizing a microelectronic workpiece includes pre-conditioning a planarizing pad for processing different types of workpieces having different feature densities and topographical patterns. For example, one embodiment of a planarizing machine can include a planarizing medium carried by a support member, a workpiece carrier configured to hold a microelectronic workpiece, and a surfacing device attached to one of the carrier or the support member. The surfacing device is positioned to transmit a non-abrasive energy, such as ultrasonic waves, a laser, and/or a water-jet, against the planarizing medium. The planarizing machine can also include a controller that is operatively coupled to the surfacing device for activating the surfacing device at appropriate moments either before or during a planarizing cycle of a microelectronic workpiece.
The controller can be a computer having a database containing instructions for causing the surfacing device to transmit the non-abrasive energy against the planarizing pad. In one embodiment, the instructions in the database activate the surfacing device when the controller receives input that a low density workpiece is to be planarized. The instructions in the database can also cause the surfacing device to transmit energy to the pad throughout at least a portion of a planarizing cycle for the low density workpiece.
In another aspect of the invention, a method for planarizing a microelectronic workpiece includes monitoring the planarity of the workpiece and causing the surfacing device to transmit energy to the planarizing pad upon an indication that the workpiece surface is at least approximately planar. For example, one embodiment of the planarizing machine can include a planarity detection system that (a) monitors a parameter indicative of planarity of the workpiece, and (b) signals the controller to activate the surfacing device at an indication of planarity. One embodiment of a planarity detection system is a device that monitors the drag force between the workpiece and the polishing pad, and estimates the onset of planarity by a step-like change in the drag force. For example, the drag force can be monitored by sensing the draw of electrical current to operate a motor that moves the table and/or the workpiece holder, and the controller can activate the surfacing device when the current draw changes in a manner that commonly occurs when the workpiece is at least approximately planar.